Resistance layers--layers of resistive material with electrodes placed in various patterns for charge injection and/or sensing--play an important role in many applications that process spatial information. If a localized region of charge is injected into the resistance layer, that charge spreads out over the resistance layer in a period of time. That charge spreading can be used to perform useful processing functions based on the spatial information of the charge distribution. For example, charge spreading can be used to form convolutions of images with various kernels, to determine the positions of spots in position sensors, and to create overlapping sensors.
The characteristics of charge spreading in the resistance layer depends on the electrical properties of the resistance layer, the elapsed time, and the properties of the charge collection electrodes attached to the resistance layer. The electrical properties of the resistance layer that determine the characteristics of the charge spreading (and therefore the uses of the resistive layer) include the sheet resistance, capacitance, leakage conductance, and the unipolar or ambipolar nature of the charge carriers. A low sheet resistance and/or a small capacitance lead to rapid charge spreading. In most practical applications, typical sheet resistances, capacitances, and dimensions result in the diffusion of charge throughout the resistance layer in milliseconds.
Collection electrodes are electrodes which are used to inject charges into the resistance layer and/or to read out the existing charge. If the collection electrodes are terminated with low impedances (such as being grounded), the collection electrodes act as sinks for the charges in the resistance layer. Consequently, charges do not diffuse significantly beyond low impedance collection electrodes. However, if the collection electrodes are terminated with a high impedance (such as being left floating), the charge spreads beyond the collection electrodes. In most applications, grounded collection electrodes are used. Position sensitive detectors, for example, determine the centroid of photoinduced charges in a resistance layer by measuring the currents flowing to two or more collection electrodes held near ground potential.
In applications using a small number of collection electrodes, low impedance terminations are practical since each collection electrode can have a dedicated external line (a line which connects the collection electrode to external circuitry) and associated processing electronics. However, in applications requiring a large number of collection electrodes, multiplexing the charge signals from many collection electrodes into the associated processing electronics becomes essential. While multiplexing greatly reduces the number of external lines and associated processing electronics, integration of the charge may be required in order to maintain an acceptable signal-to-noise ratio of the multiplexed charge signals. The term integration is taken to refer to fact that the charge induced on the resistance layer builds up during the time when the charge on the resistance layer is not being read (addressed by the multiplexer).
Multiplexing is often incompatible with low impedance terminated collection electrodes since the charge collected between each multiplexed access to the electrode must be stored. While this problem can be alleviated by increasing the sheet resistance of the resistance layer, a large sheet resistance is also often incompatible with multiplexing since an excessive amount of time would be required to remove charges built up on the resistance layer.
It is therefore highly desirable to have resistance layer structures which accomplish the charge spreading functions of previous resistance layers, but which are also more compatible with multiplexing. Such resistance layer structures should integrate charge when not being addressed by the multiplexer, and should allow rapid movement of the integrated charge into the collection electrodes when the resistance layer structure is being addressed by the multiplexer.